Patent GB-1199753-A (GB)
Title: Reference Carrier Wave Synchronizing System
IPC: H04L5/12
Filed: 1967-08-01
Abstract: 1,199,753. Automatic phase control systems; carrier frequency multiplex; frequency changing. KOKUSAI DENSHIN DENWA K.K. 1 Aug., 1967 [1 Aug., 1966], No. 35287/67. Headings H3A, H3F, H4R. In a multiplex phase-shift telegraph system operating on a plurality of harmonics of a fundamental frequency, the receiver includes an oscillator which is locked in phase to a received harmonic frequency and which provides reference frequencies for demodulating the received signals. In Fig. 1, the input signals at terminal 1 are frequency-multiplied by a factor of 4 in unit 7 and fed to a phase detector 8 fed with the frequency-multiplied output of oscillator 11 to form an automatic phase-control circuit II. The output of oscillator 11 provides the reference signals for demodulator I-#1, and is also reduced to the fundamental frequency by divider 15 which provides an input to harmonic generator 16, the output of which is filtered to provide the reference frequencies required by the other demodulators. In a modified arrangement, Fig. 2 (not shown), the outputs of the sampler units 4-1 and 4-2 in the demodulator are fed to a pair of ring modulators and a combiner to derive an error signal which is used to control the phase of oscillator 11. A further type of receiver, Fig. 4A, is used when two unmodulated signals f 0  and f n . are transmitted (Fig. 3A, not shown), the frequency difference between the two being equal to the fundamental frequency. The two frequencies are recovered by filters 208 and 209, and the beat frequency is recovered by the phase modulator 210. A similar beat frequency is derived from oscillator 214 through harmonic generator 215, filters 216 and 217, and phase modulator 219, and the two beat frequencies are compared in phase detector 211 to derive an error signal controlling oscillator 214. An additional frequency control device, shown as AFC-DEVICE (A), corrects for frequency errors in the transmission path which affect all the signals equally. A change in input frequency at input terminal 1 causes an equal change in frequency in both inputs to mixer 203 so that by selecting the difference frequency (or the sum if the changes are of opposite sign) the change is cancelled in the output, and the original transmitted frequencies are recovered at terminal 300. In a modified system, Fig. 4B, for use when a transmitted reference frequency is modulated by a timing frequency which is the source of a bit-synchronization frequency (Fig. 3B, not shown), the upper and lower sidebands are separated by filters 208 and 209, and the second input to the phase detector 211 is derived from the oscillator 214 through a frequency divider (231) the output of which is also used as an input to a frequency multiplier (232) providing the bit-synchronization frequency. Deriving beat frequencies.-Two signals from which a difference frequency is to be derived (i.e. the output of filters 208 and 209 in Fig. 4) are fed through a filter (302, Fig. 6, not shown) to derive a signal W4 which is multiplied in frequency by two (unit 304), filtered (305), and divided in frequency by two (306) to derive a signal W5; the signals W4 and W5 are fed to a phase detector (303) the output of which is filtered (307) to derive the required beat frequency in the form of a sine wave. This may be shaped into pulses by means of a bi-stable (308), a differentiator (309) and a full-wave rectifier (310). In another arrangement, Fig. 8 (not shown), the filtered input is applied directly and through a delay circuit (312) to a phase detector the output of which is filtered to give the beat frequency signal, the delay in the delay circuit being one half-cycle of the beat frequency.